Add a new event type for heart rate in AmbientContext.

Also adding a new field as ratePerMinute for events.

Bug: 305862324
Bug: 317182155
Test: e2e tested in cl/589048749 AiAi <-> DWB with this new event type and new data field

API-Coverage-Bug: 319180862

Change-Id: I950f263866b2866f8d4b7fcd68a88a25f24b8cb1